Create changelog.md

Author: dy

changelog.md

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+# 2.0.0
+
+## Breaking
+
+- ESM only. Use `import rfft from 'fourier-transform'` instead of `require()`.
+- Returns `Float64Array` instead of `Array`.
+- Returns internal buffer by default — overwritten on next call with same N. Pass an output buffer to keep results: `rfft(input, output)`.
+- Fixed off-by-one in magnitude spectrum. Imaginary components were indexed as `x[N-k-1]` instead of `x[N-k]`, shifting bins with imaginary content by one. Results are now correct.
+- Rejects N < 2 (previously N=1 returned empty array silently).
+- Removed `asm.js` entry point.
+
+## Added
+
+- `fft()` named export — returns complex DFT as `{ re, im }`, N/2+1 bins, unnormalized.
+- Optional output buffer parameter for both `rfft()` and `fft()`.
+
+## Performance
+
+- Float64Array working buffers instead of generic Array.
+- Precomputed twiddle factors and bit-reversal table, cached per size.
+- Zero-copy real part in `fft()` via subarray view.
+- ~2.7x faster than dsp.js (the original split-radix ancestor).
+- On par with fft.js (indutny) in raw transform speed.
+
+## Removed
+
+- `asm.js` — no modern engine validates `'use asm'` anymore.
+- All runtime dependencies (zero deps).